The present invention relates to the field of circuit breakers, and more particularly to multipole circuit breakers in which contact sets are paralleled in order to increase breaker capacity rating.
In the field of electrical circuit breakers, it is well known to tie the mechanisms of a plurality of electrical poles, or independent circuit paths, together. In this case, it is often desired to provide a single control lever and a trip mechanism which operates the electrical contacts in synchrony. See, U.S. Pat. Nos. 5,565,828; 5,557,082, 4,492,941, and 4,347,488, expressly incorporated herein by reference.
A single pole circuit breaker is a device that serves to interrupt electrical current flow in an electrical circuit path, upon the occurrence of an overcurrent in the circuit path. On the other hand, a multipole circuit breaker is a device which includes two or more interconnected, single pole circuit breakers which serve to substantially simultaneously interrupt current flow in two or more circuit paths upon the occurrence of an overcurrent in any one circuit path.
In a multipole circuit breaker, typically the poles switch independent phases of AC current. Thus, two-pole and three-pole breakers are well known. In these systems, each pole is provided with a current sensing element to generate a trip signal, so that an overload on any phase circuit is independently sensed. In the event that an overload occurs, all of the phase circuits are tripped simultaneously. A manual control lever is provided which operates the phase circuits synchronously as well.
Conventional multipole circuit breaker arrangements thus include a trip lever mechanism associated with each pole of the multipole circuit breaker. Each trip lever includes a portion for joining it to adjacent trip levers. If any pole is tripped open by an overcurrent, the breaker mechanism of that pole causes the trip lever to pivot about its mounting axis. The pivotal motion of one lever causes all the interconnected trip levers to similarly pivot. Each lever may include an arm for striking the armature or toggle mechanism of its respective pole, and causing each pole to be tripped open.
In order to increase the capacity of a circuit breaker system, it has been proposed to parallel a set of contacts, each of which might be insufficient alone to handle the composite load. Thus, by paralleling two single pole circuit breaker elements, a higher capacity circuit breaker may be achieved. However, the art teaches that, preferably, a single contact set is provided having a larger surface area and greater contact force in order to handle a larger load. These larger load-handling capacity devices are typically dimensionally larger than lower load carrying designs. This is because, in part, many elements within a circuit breaker scale in size in relation with current carrying capacity, including the lugs, trip elements, trip mechanism, contacts and breaker arm.
In designing a trip element or system, the type of load must be considered. There are two main classes of trip elements; thermal magnetic and magnetohydraulic. These differ in a number of characteristics, and typically have different application in the art.
However, where such contact parallelization is employed, the contact ratings of the breaker should be derated from the sum of current carrying capacity of each of the contact sets. This is because a contact set having a lower impedance than others will xe2x80x9chogxe2x80x9d the current, and may thus see a significantly greater proportion of the total current than 50%, resulting in overheating, and possible failure. Therefore, the art typically teaches that a pair of paralleled contact sets are derated, by for example about 25%, to ensure that each component will operate within its safe design parameters. Further, the contact resistance of a switch may change significantly with each closure of the switch. In parallel contact systems, it is known to employ both unitary thermal magnetic and multiple parallel-operating trip elements in multipole breakers. Thus, it is possible to design a circuit breaker with a specially designed trip element that controls an entire breaker system, or to parallel two entire breaker circuits of a multipole arrangement. In the later case, in order to equalize the current as much as possible between the circuits, a current equalization bar has been proposed. However, this does not compensate for unequal contact resistance, and nuisance tripping of the circuit breaker results when the unequal division of the current has caused enough current to pass through one of the current sensing devices to cause it to trip its associated mechanism.
Attempts have been made in thermal-type breakers to parallel the sets of contacts of a multipole breaker to achieve increased maximum current rating. In one case, exemplified by model QO12150 from Square-D Corp., a unitary thermal magnetic trip element was employed as a trip element for a set of two parallel contact sets, with a connecting member to trip both contact sets at the same time. In this case, the trip dynamics were defined by the thermal-magnetic trip element, and careful calibration of the thermal element was required. This design provided both contact sets within a common housing. Thus, while the internal parts were common with non-multipole arrangements, the housing itself was a special multipole breaker housing. The parallel breaker is housed in a shell that differs from single pole housings, with the parallel poles in a common space.
One typical known system is disclosed in U.S. Pat. No. 4,492,941, expressly incorporated herein by reference, provides electromagnetic sensing devices that are electrically connected at one of their ends to the load terminals. The load terminals are electrically connected in parallel with each other. A plurality of electromagnetic sensing devices are electrically connected at their other ends to each other and are electrically connected to all of the movable contacts which are themselves all electrically connected together. The stationary contacts are connected to line terminals that are also electrically connected in parallel with each other. Thus, the electromagnetic sensing devices are connected in parallel at both of their ends and the contact sets are also connected in parallel at both of their electrical ends, while the electromagnetic sensing devices, on the one hand, and the contact sets, on the other hand, are also in series with each other, thus seeking to equally divide the current among all of the electromagnetic sensing devices, even though the current may not be equally divided among all of the relatively movable contacts, because of varying contact resistances.
Another attempt to increase current carrying capability by paralleling contact sets using magnetohydraulic trip elements employed two parallel trip elements, each set for a desired derated value corresponding to half of the total desired current carrying capacity. For example, two 100 Amp breakers were paralleled (using a standard multipole trip bar) to yield a 150 Amp rated breaker, with 175% trip (about 250 Amps) rating, meeting UL 1077. The parallel set of breakers employed two side-by-side single breaker housings, with slight modifications, and thus did not require new tooling for housings and contact elements.
In this later case, it is difficult to comply with UL 489, which requires that the breaker trip at 135% maximum of rated capacity and 200% of rated capacity within 2 minutes, and that the breaker be capable of handling the specified loads without damage. For example, if the maximum expected deviation in contact resistance of the contact sets (which changes each time the contact is closed) could cause a current splitting ratio of 60%/40%, then in order to ensure reliable trip at 135% of total rated capacity, each trip element must be designed to trip at about 120% of rated capacity, which would lead to unreliability and nuisance trips because of insufficient margin.
Notwithstanding the foregoing attempts, it has heretofore been considered difficult to employ magnetohydraulic circuit breakers in parallel contact multipole breakers with relatively low overcurrent thresholds, such as that imposed by UL 489, especially for use in load environments with high peak to average load ratios, because the maximum expected currents would result in nuisance trips.
A main advantage of parallel contact circuit breakers is that these may employ many parts in common with lower current carrying single pole devices. It is thus often economically desirable to increase the current carrying capacity of circuit breakers by modifying as little as possible, existing circuit breakers. Toward this end, it has been proposed that the amount of current carrying capacity may be almost doubled by placing two single pole circuit breakers side-by-side (or almost tripled by using three side-by-side) and connecting the line terminals together and likewise connecting the load terminals together.
Commercial circuit breaker manufacturers generally manufacture a complete product line composed of a number of breaker sizes, each one covering a different (although sometimes overlapping) operating current range. Each breaker size typically has required its own component and case sizes. In general, each component and case size combination is useful in circuits having only a single current rating range. The need to have a different set of component and case sizes for each current rating has added to the overall cost of breakers of this general type.
As discussed above, there are two common types of trip elements for circuit breakers. A first type, called a thermal magnetic breaker, provides a thermal portion having a bimetallic element that responds to a heat generated by a current, as well as a solenoid to detect magnetic field due to current flow. Typically, the thermal element is designed to trigger a trip response at a maximum of 135% average of rated capacity, and the magnetic element responds quickly (within milliseconds) at 200% of rated capacity. The thermal portion of the breaker controls average current carrying capability, by means of thermal inertia, while the magnetic element controls dynamic response. This design seeks to provide adequate sensitivity while limiting nuisance trips. However, such thermal magnetic designs typically require calibration of the thermal trip mechanism for precision, and tuning of dynamic response is difficult. Further, the thermal element incurs a wattage loss. The operation of the thermal element is also sensitive to ambient temperature, since the heating of the bimetallic element by the current flow is relative to the ambient temperature. See, U.S. Pat. Nos. 3,943,316, 3,943,472, 3,943,473, 3,944,953, 3,946,346, 4,612,430, 4,618,751, 5,223,681, and 5,444,424.
A second type of trip element is called a magnetohydrodynamic or magnetohydraulic breaker. See, U.S. Pat. Nos. 4,062,052 and 5,343,178. In this element, the current passes through a solenoid coil wound around a plastic bobbin, acting on static pole piece and a movable armature. Within the solenoid coil is a moveable magnetically permeable core, which is held away from the pole piece in a damping fluid, e.g., a viscous oil, by a spring. As a static current through the coil increases, the core is drawn toward the pole piece through the viscous fluid, resulting in a nonlinear increase in force on the armature, which lies beyond the pole piece, as the moveable core nears contact with the pole piece. Thus, as the moveable core is pulled toward the pole piece, the magnetic force on the armature suddenly increases and the armature rapidly moves. In this case, it is primarily the spring constant of the spring which controls the precision of the trip element, and thus a final calibration is often unnecessary given the ease of obtaining precision springs. In the event of a dynamic current surge, the core is damped by the fluid, and thus does not rapidly move toward the pole piece, resulting in a dynamic overload capability, determined by the viscosity of the damping fluid, and thus avoiding nuisance trips. The armature is typically counterbalanced and may be intentionally provided with an inertial mass to provide further resistance to nuisance trips.
Nuisance tripping is a problem in applications where current surges are part of the normal operation of a load, such as during motor start-up or the like. For example, starting up of motors, particularly single phase, AC induction types, may result in high current surges. Motor starting in-rush pulses are usually less than six times the steady state motor current and may typically last about one second, but may be 10 or more times the steady state current. In the later case, a breaker may revert to an instantaneous trip characteristic, because the magnetic flux acting on the armature is high enough to trip the breaker without any movement of the delay tube core or heating of the thermal element, depending on the design. One way to address this problem is by increasing the distance between the coil and armature.
A second type of short duration, high current surge, commonly referred to as a pulse, is encountered in circuits containing transformers, capacitors, and tungsten lamp loads. These surges may exceed the steady state current by ten to thirty times, and usually last for between two to eight milliseconds. Surges of this type will cause nuisance tripping in conventional delay tube type electromagnetic circuit breakers. This problem may be addressed by increasing the inertia of the trip element or by other means. See, U.S. Pat. Nos. 4,117,285, 3,959,755, 3,517,357, and 3,497,838, expressly incorporated herein by reference.
The applicants have found that a single magnetohydraulic trip element can advantageously be used to provide desired trip dynamics in a circuit breaker by passing all current from a set of parallel contact sets through a unitary trip element, and providing a multipole trip arm triggered by the unitary trip element which trips the parallel contact sets simultaneously.
The preferred design employs parallel circuit breaker poles each having a trip mechanism, switch contacts and a housing, which share most components in common with a single pole circuit breaker in the same xe2x80x9cfamilyxe2x80x9d, thus reducing required number of inventoried parts and engineering costs. The trip element of the preferred design, however, differs from single pole designs, being configured for the desired ratings and dynamic response, and portions of the housing between adjacent poles are modified for common access to electrical terminals to bridge the load and to provide a standard type multipole trip bar. The magnetohydraulic trip element, which is preferably a 150 Amp element with desired dynamic trip characteristics, sits asymmetrically in one of the pole housings within a standard frame, in the normal trip element position, and actuating a standard armature.
The external lugs of each poles are made electrically parallel by placing a conductive bar therebetween. This also serves the visual function of alerting the installer as to the electrical function of the breaker, which is similar to a multipole breaker that is not paralleled. Internally, one set of lugs are connected together with conductive straps to one end of the magnetic coil. The other end of the magnetic coil is connected with conductive straps to each of the contact arms. In order to provide physical access for these connections, a portion of each of the common walls of the breaker pole housings are machined to form an aperture or portal therebetween.
The modifications to the standard single pole housing are minimized; other than the portal in the common wall between the poles, the only other modifications are, for example, an arcuate slot for a common trip mechanism, and an arcuate slot for an internal linkage of the manual switch handles. In the preferred embodiment, however, the handles are linked externally by a crossbar, which fits between the handles and causes them to move in unison. In this way, the standard mountings for the handle, pivot axis of the moveable contact bar, stationary contact, and arc chute and slot motor are unaffected. Further, the safety factors of the design remain relatively intact.
A preferred design provides two parallel switch poles with a design rating of 100 Amps each, in a housing 2.5 inches long, 0.75 inches wide, and 2 inches deep, with electrical contact bolts on 2 inch centers. The resulting parallel multipole design with a rating of 150 Amps therefore fits within a form factor of 2.5 by 1.5 by 2 inches, a substantial improvement over prior 150 Amp rating circuit breakers.
It should be seen that the form factor may be varied according to the present invention, for example other standard size circuit breakers which may be formed as multipole parallel contact breakers are, for example, 2 inches long, by 0.75 inches wide, by 1.75 inches deep (e.g., 50 Amp rating) and 7.25 inches long by 1.5 inches wide by 3 inches deep (e.g., 250 Amp rating).
The present invention may incorporate other known circuit breaker features, such as a mid-trip stop for the manual control lever or other trip indicators, and indeed may be formed into a traditional multipole design with parallel sets of contacts for each of multiple switch poles.
It is also seen that, while the preferred embodiments employ housing parts which are common in essential design with single pole designs, that this is not a limitation on the operability of the inventive design.
It is therefore an object of the invention to provide a magnetohydrodynamic circuit breaker which has a low average overcurrent trip capability with good nuisance trip immunity.
It is also an object of the present invention to provide a circuit breaker having a high current rating and a small form factor.
It is a further object of the invention to provide a circuit breaker having a set of parallel contacts, driven by a trip mechanism, wherein all of the contact sets are tripped by a common magnetohydrodynamic trip element.
These and other objects will be apparent from an understanding of the preferred embodiments.